X-ray scanning system

ABSTRACT

An x-ray scanning system includes an x-ray source that produces a collimated fan beam of incident x-ray radiation. The system also includes a chopper wheel that can be irradiated by the collimated fan beam. The chopper wheel is oriented with a wheel plane containing the chopper wheel substantially non-perpendicular relative to a beam plane containing the collimated fan beam. In various embodiments, a disk chopper wheel&#39;s effective thickness is increased, allowing x-ray scanning with end point energies of hundreds of keV using relatively thinner, lighter, and less costly chopper wheel disks. Backscatter detectors can be mounted to an exterior surface of a vehicle housing the x-ray source, and slits in the disk chopper wheel can be tapered for more uniform target irradiation.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/527,566, filed May 17, 2017, which is the U.S. National Stage ofinternational Application No. PCT/US2015/061952, filed Nov. 20, 2015,which designates the U.S., published in English, and claims the benefitof U.S. Provisional Application No. 62/084,222, filed on Nov. 25, 2014and U.S. Provisional Application No. 62/082,321, filed on Nov. 20, 2014.The entire teachings of the above applications are incorporated hereinby reference.

BACKGROUND

X-ray backscatter (BX) imaging has been used since the 1980's fordetecting concealed contraband, such as drugs, explosives, and weapons.The BX imaging technique differs in fundamental ways from thetraditional transmission (TX) x-ray method of searching for contraband.The TX method works by creating an image of the x-rays that aretransmitted through the target. Transmission images are created by a fanor cone beam of x-rays that pass through the target to a pixelated x-raydetector. The size of the individual detector elements determines thepixel resolution in the transmission image.

In contrast to TX images, backscatter images are created by scanning thetarget with a pencil beam of x-rays and measuring the intensity of thebackscattered x-rays as a function of the position of the pencil beam onthe target. Both x-ray scanners and backscatter detectors have been usedon mobile platforms.

Scanning pencil beams of x-rays are created in several distinct ways,which have in common the creation of a fan beam of x-rays that isperiodically cut (chopped) by a rotating slot or aperture created by avariety of means. The spatial resolution in the image is determined bythe cross sectional area of the resulting scanning beam at the pointthat it strikes the target object being imaged.

SUMMARY

The apertures used to create the pencil beams for BX imaging aretypically formed through materials that attenuate the initial x-ray beamby at least a factor of 10⁸. The needed thickness of the material,typically tungsten or lead, increases rapidly as the end-point x-rayenergy increases. For example, only 3 mm of tungsten are needed for anend-point x-ray energy of 120 keV, but 12 mm of tungsten are needed whenthe end-point energy doubles to 240 keV. This required increase inthickness leads to greater system weight and chopper wheel moment ofinertia, with related increases in system complexity and cost. Thesefactors effectively limit the range of end-point x-ray energies that canreasonably be used.

Many embodiment systems described herein can use scanning x-ray beamswith energies greater than about 100 keV. These scanning beams can becreated by a rotating, x-ray opaque disk with radial slots, for example.

Moreover, in relation to detection of backscattered x-rays, prior-art,box-type, backscatter detectors are bulky and cannot therefore be easilymounted to the exterior of a vehicle for mobile imaging systems,particularly if the system needs to be used covertly. Therefore,existing backscatter detectors need to be concealed within the vehicleenclosure or to be stored in an external recessed cabinet. Both of theseoptions require extensive modifications to the vehicle enclosure. Itwould therefore be advantageous, from a cost and simplicity viewpoint,to have detectors with a thin enough profile that they can be mounteddirectly onto the exterior of the vehicle enclosure, withoutmodification to the enclosure.

In one embodiment system and corresponding method, an x-ray scanningsystem includes an x-ray source configured to produce a collimated fanbeam of incident x-ray radiation. The system also includes a chopperwheel that is configured to be irradiated by the collimated fan beam.The chopper wheel is oriented with (i.e., in or parallel to) a “chopper”plane containing the chopper wheel (also referred to herein as a “wheelplane”, where the wheel plane is substantially non-perpendicularrelative to a plane containing the collimated fan beam of incidentradiation (also referred to herein as a “beam plane”).

The system can have an angle of less than 30° between the wheel planeand the beam plane. This angle can also be less than 15°.

The chopper wheel can be a disk with a rim and a center. The disk caninclude one or more radial slits extending toward the rim of the diskand toward the center of the disk. One or more of the slits can beconfigured to pass x-ray radiation from the collimated fan beam. The oneor more slits can be tapered slits having greater width toward the rimof the disk than toward the center of the disk. Furthermore, the chopperwheel can include chamfering on at least two edges, or on all edges, ofthe one or more slits. The one or more slits can be tapered and alsoinclude chamfering on edges.

The x-ray source can be further configured to produce the collimated fanbeam of incident x-ray radiation with end point x-ray energies in arange between about 50 keV and 500 keV. Furthermore, the endpoint x-rayenergies can be in a range between about 200 keV and 250 keV.

The system can also include one or more backscatter detectors configuredto detect x-ray radiation backscattered by objects irradiated by theincident radiation having passed through the chopper wheel. The one ormore backscatter detectors can be mounted to an external surface of avehicle.

In another embodiment, a method and corresponding system for x-rayscanning includes producing a collimated fan beam of incident x-rayradiation. The method also includes effecting rotation of a chopperwheel that is configured to be irradiated by the collimated fan beam.The rotation of the chopper wheel is effected in a rotation wheel planethat is substantially non-perpendicular relative to a beam planecontaining the collimated fan beam of incident radiation.

Effecting rotation of the chopper wheel can include causing the rotationwith an angle between the wheel plane and the beam plane being less than30°. Furthermore, in certain embodiments, this angle can be less than15°.

Effecting rotation of the chopper wheel, which results in effectingscanning of an x-ray beam incident at a scanning target, can furtherinclude using a disk chopper wheel with a rim, a center, and one or moreradial slits extending toward the rim of the disk and toward the centerof the disk. The one or more slits can be configured to pass x-rayradiation from the collimated beam. The rotation can be effected usingthe disk chopper wheel with one or more tapered slits having greaterwidth toward the rim of the disk than toward the center of the disk.Effecting rotation of the chopper wheel can include using the diskchopper wheel with chamfering on at least two edges or on all edges ofthe one or more slits. Chopper wheels can include slits that are taperedand can also include chamfering on edges of the slits.

Producing the collimated fan beam can include producing x-rays withendpoint energies between about 50 keV and 500 keV. Furthermore, the endpoint energies can be between about 200 keV and about 250 keV.

The method can also include detecting x-ray radiation backscattered byobjects irradiated by the incident radiation having passed through thechopper wheel. Detecting the backscatter x-ray radiation can includeusing one or more backscatter x-ray detectors mounted to an externalsurface of the vehicle that houses the chopper wheel.

In yet another embodiment, an x-ray scanning system includes an x-raysource configured to produce a collimated fan beam of incident x-rayradiation. The system also includes one or more backscatter detectorsmounted to an exterior surface of a vehicle. The one or more backscatterdetectors can be fixedly mounted to the exterior surface of the vehicle,and the detectors can be wavelength-shifting fiber (WSF) detectors.

The system can also include a chopper wheel that is configured to beirradiated by the collimated fan beam. The chopper wheel can be orientedwith a wheel plane containing the chopper wheel substantiallynon-perpendicular relative to a beam plane containing the collimated fanbeam of incident x-ray radiation. The chopper wheel can be oriented witha plane containing the chopper wheel (i.e., “wheel plane”) substantiallyperpendicular relative to a plane containing the collimated fan beam ofincident radiation (i.e., “beam plane”).

In still a further embodiment, an x-ray scanning system includes anx-ray source configured to produce a collimated fan beam of incidentx-ray radiation. The system also includes a disk chopper wheel that isconfigured to be irradiated by the collimated fan beam. The disk chopperwheel is oriented with a wheel plane containing the disk chopper wheelthat is substantially perpendicular relative to a beam plane containingthe collimated fan beam of incident radiation. The disk chopper wheelincludes one or more tapered radial slits extending toward a rim of thedesk and towards a center of the disk, with the one or more taperedslits having greater width toward the rim of the disk than toward thecenter of the disk and being configured to pass x-ray radiation from thecollimated fan beam.

Advantages of certain embodiments include extending the applicability ofthe scanning chopper wheel disk to significantly higher x-ray energies.With much higher x-ray energies, significantly thicker targets can beimaged. Various embodiments can substantially reduce the weight ofchopper disk BX inspection systems operating in the 200 keV to 500 keVrange. Furthermore, the cost of chopper disk BX inspection systems thatoperate in the 200 keV to 500 keV range can be significantly reduced.

Furthermore, some embodiment systems can include detectors with a thinenough profile that they can be mounted directly onto the exterior ofthe vehicle enclosure, without modification to the enclosure.

BRIEF DESCRIPTION

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic diagram illustrating an embodiment x-ray scanningsystem mounted within a vehicle.

FIG. 2 is a more detailed illustration of the chopper disk in thescanning system shown in FIG. 1.

FIG. 3 is an illustration of an embodiment x-ray scanning system with asubstantially non-perpendicular angle between the chopper disk and x-rayfan beam.

FIG. 4 is a more detailed illustration showing the orientation of thechopper disk in FIG. 3 with respect to the incident fan beam.

FIG. 5 is a graph showing an effective thickness multiplier as afunction of angle θ between a chopper disk and incident x-ray fan beam.

FIG. 6 is a table showing the effective thickness of tungsten needed toprovide a factor of 10⁸ attenuation of an incident x-ray beam.

FIG. 7 is a graph showing the tungsten thicknesses listed in the tableof FIG. 6.

FIG. 8 illustrates a chopper disk with a chamfered slit edge.

FIG. 9 illustrates a chopper disk with tapered slits.

FIG. 10 illustrates a chopper disk, oriented substantially perpendicularto an x-ray fan beam, that can include tapered slits or chamfered slitsor be used in an embodiment system that includes backscatter detectorsmounted externally on a vehicle.

FIG. 11 illustrates a low-profile backscatter detector mountedexternally on a van.

FIG. 12 is a flow diagram illustrating an embodiment method of x-rayscanning.

DETAILED DESCRIPTION

A description of example embodiments of the invention follows.

Rotating disks were used in the earliest x-ray imaging systems designedin 1972. Initially, the rotating disk was used to create a digitaltransmission x-ray imaging system. Backscatter imaging was eventuallyadded by incorporating additional scatter detectors in the backdirection. Backscatter imaging using chopper disks has been used sincethe 1980's to create scanning beams of x-rays with end-point energies inthe 120 keV to 160 keV range. The method has not been practical at muchhigher energies because the needed thickness (and therefore weight) ofthe rotating disk increase rapidly with energy.

Disclosed herein are embodiment x-ray scanning systems that can beextended to operate at much higher x-ray energies of at least 500 keVwhile controlling thickness and weight of chopper disks.

FIG. 1 is a schematic diagram illustrating an embodiment x-ray scanningsystem 100 mounted within a vehicle 102. The system 100 includes achopper disk 104, as well as an x-ray source (not shown in FIG. 1), bothof which are illustrated further in figures described hereinafter. Thechopper disk 104 is a chopper wheel that is substantially disk-shaped,as further illustrated in FIG. 2. Alternatively, in other embodiments, achopper wheel need not be perfectly circular or completely disk-shaped,so long as it can be rotated to scan an x-ray beam, as further describedhereinafter.

A scanning pencil beam 130 of x-rays is swept from the system 100 overthe car 106 to scan the car for contraband. Backscattered x-rays 108 arescattered from the car, and an external detector 110 is mountedexternally on the vehicle 102 and detects the scattered x-rays 108. Theexternal detector 110 is mounted in a fixed manner to the vehicle, suchthat detector folding or removal or other reconfiguration is notnecessary when the vehicle is driven. The external detector 110 isdescribed further hereinafter in the description of FIG. 11.

FIG. 2 is a front view that further illustrates the chopper disk 104 ofFIG. 1 in greater detail. The embodiment chopper disk 104 includes aband of attenuating material 212, which can be lead or tungsten, forexample. Alternatively, other attenuating materials can also be used.The attenuating material of the chopper disk includes a series of radialslits 214 therethrough that allow x-rays to pass through the slits. Across section of a fan beam 328 of x-rays is illustrated intersectingwith the disk 104. The radial slit 214 is configured to pass x-rayradiation of the collimated fan beam 328, as further illustrated in FIG.3. In particular, the intersection of the fan beam 328 with a radialslit 214 as the disk is rotated allows a pencil beam 130 of x-rays(illustrated in FIGS. 1 and 3) to pass through the disk 104.

The pencil beam is illustrated, in FIG. 1, as pencil scanning beam 130,and also in other figures described hereinafter. As the disk rotates,the pencil beam of x-rays scans within the plane of the incident fanbeam, with the scan direction defined by the line of sight from thex-ray source focal spot through the illuminated slit 214. If the fanbeam is vertical, then the beam scans up and down as the disk rotates.Alternatively, if the fan beam is horizontal, for example, the beamscans from side to side as the disk rotates.

The material making up the bulk of the chopper disk can be constructedof aluminum, for example. Aluminum is advantageous because it is lighterthan lead or tungsten. Since the fan beam 218 only intersects withattenuating material 212, the material making up the bulk of the diskneed not be as opaque toward x-rays as lead or tungsten are for theattenuating material 212, for example. Alternatively, other materialsbesides aluminum can be used for the outer perimeter. Furthermore, asolid tungsten disk with slits machined into it can also be used inother embodiments.

In the embodiment of FIG. 2, four radial slits 214 in the lead disk withslit edges defined by tungsten “jaws” create four sweeping pencil beamsper rotation of the disk 104. However, in other embodiments, differentnumbers of radial slits can be used, and the slits can be oriented suchthat the fan beam intersects with only one slit at a time as the diskrotates.

FIG. 3 illustrates how the chopper disk 104 can be used in an embodimentx-ray scanning system 300. An x-ray tube 320 generates a broad beam 322of x-rays. The broad beam 322 irradiates an opaque plate 324 that isopaque to x-rays and has a plate slit 326. The opaque plate 324 with theplate slit creates an x-ray fan beam 328. The x-ray tube 320 and opaqueplate 324 form an x-ray source 319 that is configured to produce thecollimated fan beam 328 of incident x-ray radiation that is incident onthe chopper disk 104. In other words, the chopper wheel, or disk in theembodiment of FIG. 3, is configured to be irradiated by the collimatedfan beam of incident x-ray radiation. In some embodiments, the x-raysource is configured to produce the collimated fan beam of incidentx-ray radiation with end-point energies in a range of between about 50keV and about 500 keV, or between about 200 keV and about 250 keV, forexample. The x-ray source 319 and chopper disk 104 form a system 300that can be used for x-ray scanning.

The fan beam 328 intersects the chopper disk 104, which is oriented in adisk plane that is substantially non-perpendicular with respect to abeam plane in which the fan beam 328 is oriented. In other words, thechopper disk is oriented with the disk plane that is substantiallynon-perpendicular, at an angle Θ, relative to the beam plane containingthe collimated fan beam of incident radiation. In some embodiments, theangle Θ is less than 30°, for example. In other embodiments, the angle Θis greater or less than this range, such as less than 15°, for example.This orientation is further illustrated in FIG. 4.

Continuing to refer to FIG. 3, when one of the equally-spaced radialslots is in the fan beam, a scanning pencil beam 130 is created. As thedisk 104 rotates, the radial slits 214 chop the fan beam 328 into thescanning pencil beam 130.

Referring again to FIG. 3, the scanning beam 130 is scattered from asuitcase 332 on a conveyor belt 334. The target suitcase 332 causesbackscattered x-rays 336 to be scattered toward backscatter detectors310. The detectors 310 record intensity of scattered x-rays 336 as afunction of the beam position on the target suitcase. As the targetsuitcase moves through the plane of the scanning pencil beam 130 on thebelt 334, a two-dimensional backscatter image of the suitcase target isobtained. Other targets can include cars, as illustrated in FIG. 1, orany other object or material. As further described hereinafter, thenon-perpendicular angle between the chopper disk and the fan beam allowsthe chopper disk to attenuate higher energy x-rays with substantiallylower disk weight and cost.

FIG. 4 illustrates the orientation of the fan beam 328 and chopper disk104 in greater detail. The x-ray tube 320 is oriented with an axis inthe Y direction. The fan beam 328 is oriented in the X-Z plane (the X-Zplane contains the fan beam 328). The plane of rotation of the chopperdisk lies at an oblique non-perpendicular angle Θ to the X-Z plane Thescanning pencil beam 130 also is scanned in the X-Z plane, i.e., thebeam plane, as the chopper disk rotates. The chopper disk 104 includes arim 440 and center 438, and the slits 214 are oriented to extendradially toward the rim and center. The chopper disk 104 is rotated bymeans of a motor 442.

The chopper disk 104 is not oriented in either the X-Z plane or the X-Yplane, but, rather, in a disk plane that is at an angle Θ with respectto the beam plane (X-Z plane) of the fan beam 328. The disk plane canalso be referred to as a plane of rotation (or rotational plane) of thechopper disk 104, because the disk remains parallel to this plane as itrotates. The disk plane can be parallel to the X axis. By positioningthe plane of the rotating disk at an acute (substantiallynon-perpendicular) angle Θ to the plane of the fan beam, the actualthickness of the disk can be reduced by a factor F=1/sin (θ) whilekeeping the disk's effective thickness the same. As used herein,“substantially non-perpendicular” indicates that the angle Θ is smallenough to increase effective thickness significantly, such as increasingeffective thickness by more than 25%, more than 50%, more than 100% (aneffective thickness multiplier of 2), more than 200%, or more than 400%.

FIG. 5 is a graph showing an effective thickness multiplier plotted as afunction of θ. For example, with the included angle reduced from 90° to15°, the effective thickness has increased by an effective thicknessmultiplier factor of 4. A 3 mm thick tungsten disk then has theradiational stopping power of a 12 mm disk.

FIG. 6 is a table showing the effective thickness of tungsten needed toprovide a factor of 10⁸ attenuation of an incident x-ray beam. As shownin the table, the end-point x-ray energy of a scanning system with a 3mm thick disk is increased from 120 keV to about 240 keV. As a furtherexample, a very highly penetrating 400 keV end-point energy scanningpencil beam can be created by using a 5.5 mm thick disk at an includedangle θ of 10°.

FIG. 7 is a graph showing the tungsten thicknesses and x-ray end pointenergies listed in the table of FIG. 6.

Weight reduction is useful for hand-held x-ray imaging devices also, butit is also a factor for all uses of the rotating disk x-ray scanningmethod. This is because weight reduction is accompanied by costreduction, not only of the disk itself but also the ancillary equipment,such as a driving motor and support bearings. A dramatic reduction inweight can be achieved with a disk with a grazing angle of incidence.For practical applications requiring disks with radii in the 25 cmrange, the weight reduction can be more than 50 pounds, and the cost ofgoods can be lowered by thousands of dollars. For example, for a 225 keVx-ray system with an 18″ diameter disk, the weight reduction would be23.7 kg (52 lbs) compared with an existing perpendicular illuminationdisk.

In addition to saving weight, the materials cost savings are alsosubstantial. For the hand-held system, the savings from the reducedtungsten material would be approximately $40 USD at today's values. Forthe 18″ disk in the 225 keV system, however, the savings from thereduced amount of tungsten can be close to $3,000 USD at today's values.In addition to a smaller, lighter chopper disk, a smaller motor can alsobe used to spin the disk, due its greatly reduced moment of inertia.Alternatively, the same motor can be used with a greatly reduced spin-uptime to reach the 1,800-2,500 rpm rotation speed typically used inbackscatter imaging systems. Another advantage of the reduced moment ofinertia of the disk is a large reduction in gyroscopic effects, whichcan cause unwanted torques on the system when the system is moved.

Thus, substantially non-perpendicular chopper disks can facilitate x-rayscanning with end-point x-ray energies in a range above 500 keV.Furthermore, x-ray scanning with energies in a range of between about 50keV and about 500 keV can be facilitated. For example, x-ray scanningwith energies in a range of between about 200 keV and about 250 keV canbe facilitated. As used herein in reference to x-ray energy, “about”indicates an energy tolerance of ±10%.

In order to allow the x-rays to pass through the slits of the chopperdisk with an acute included angle, the two ends of the slit can beheavily chamfered.

FIG. 8 illustrates a chopper disk 804 with a radial slit 814 that has achamfered edge 816 at the end of the slit. The edge 816 extends towardthe rim of the disk. While not shown in FIG. 8, the other end of theslit, which extends toward the center of the disk 804, can also bechamfered similarly. Furthermore, the longer edges of the slit can alsobe chamfered, in order to allow the beam to pass cleanly through theslit as the disk rotates, at all intersection points of the slit withthe incident fan beam. Thus, all edges of the slits, including fouredges, can be chamfered. While slits need not be chamfered in allembodiments, chamfering is useful, especially when included anglesbetween the fan beam and chopper wheel are smaller, and fan beam x-rayswould otherwise be attenuated by passing through an edge of a plate.

FIG. 9 illustrates another aspect of embodiments disclosed herein,namely tapering of the radial slits to maintain a constant beamintensity as the beam sweeps through the slit. Tapering refers to thevariable width of slits. In FIG. 9, a chopper disk 904 includes slits926 that are tapered, with the width of the slits increasing from thecenter 938 of the disk toward the rim 940 of the disk. In other words,the slits 926 have greater width toward the rim of the disk than towardthe center of the disk. The tapering of the slits is designed so thatthe solid angle of the slits, as viewed from a focal spot 942 of thex-ray tube, remains approximately constant through the scan. A firstorder equation describing this condition is Equation (1):

$\frac{A_{1}}{D_{1}^{2}} = \frac{A_{2}}{D_{2}^{2}}$

where A₁ and A₂ are the areas of the region where the slit 926 overlapswith the incident illuminating fan beam 328 when the slit is at thecenter and end of the scan, respectively, and D₁ and D₂ are therespective distances between the x-ray source focal spot (FS) and thecenters of the overlap areas A₁ and A₂, when the slit is at the centerand end of the scan, respectively. The advantage of the tapering isemphasized by considering a reduction in beam intensity at the extremesif the slits are not tapered. The ratio of beam intensity when theuntapered slit is at the center of the fan beam to the intensity when atthe end is shown in Equation (2):

$\frac{I_{2}}{I_{1}} = \frac{D_{2}^{2}}{D_{1}^{2}}$

For an 18″ diameter disk 12″ away from the focal spot, for example, theintensity I₂ at the extremes will only be about 64% of the intensity I₁at the center of the scan. This causes issues with the backscatterimaging, as the images appear darker and noisier at the extremes of thescan. By using tapered disks with the slit tapering designedspecifically so that A₁ and A₂ satisfy Equation (1), the intensities I₁and I₂ can be made to be equal, and the image brightness and noisecharacteristics are more uniform across the scan.

FIG. 10 illustrates how some embodiments disclosed herein can usechopper disks oriented substantially perpendicular to x-ray fan beams.As used herein, “substantially perpendicular” indicates perpendicular towithin a range in which effective thickness of a chopper disk is notincreased significantly, such as not more than 25%, not more than 10%,or not more than 5%. In FIG. 10, the fan beam 328 is oriented in the X-Zplane, while a chopper disk 1004 is situated, and undergoes rotation, ina disk plane that coincides with (or is parallel to) the X-Y plane.Radial slits 1014 are chamfered along the long edges, as described inconjunction with FIG. 10. In other embodiments, the perpendicularorientation illustrated in FIG. 10 can be used for chopper disks havingtapered slits, for example. In yet other embodiments, the perpendicularorientation may be used in a mobile x-ray scanning system with externaldetectors, as illustrated in FIG. 1, for example.

External Detectors

X-ray backscatter imaging was first implemented on a mobile platform inthe early 1990's. The first system operated at 450 keV. Systems of thattime period were fitted with an extendable boom, which when deployed,provided either a beam stop for intercepting the radiation on the farside of the object being imaged, or a transmission detector, so that thesystem could provide both backscatter and transmission x-ray images.

Furthermore, some systems included a large internal chopper wheel tocreate the 450 keV “flying spot” pencil beam of x-rays to scan up anddown as the system is slowly driven past the object being scanned. Thebackscatter detectors on some mobile systems consisted of hollow boxeslined with scintillator screen, and viewed with Photo-Multiplier Tubes(PMTs), which detect the light emitted from the scintillator when anx-ray gets absorbed. Eight of these detector boxes, for example, werepositioned in an external cabinet built into the mobile scanner truckhousing. It should be noted that these truck enclosures had to beheavily modified in order to house the backscatter detectors, which areabout 14″ wide, 12″ deep, and 60″ high. In the original design, theupper detector boxes could be hydraulically lowered so that thedetection efficiency of the backscattered x-rays is increased whensmaller vehicles are scanned. In later designs, the detectors were notdeployable, and all eight detector boxes remained fixed permanently inthe vertical, stowed position.

In almost all the existing backscatter systems, the backscatterdetectors are of a similar standard design. These consist of hollowplastic or aluminum boxes lined with scintillator screen, such asgadolinium-oxy sulfide (GdOS) or barium fluoro-chloride (BaFCl2) with afront face transparent to x-rays to let the radiation through.Photo-multiplier tubes, typically located at the rear of the boxes, areused to detect the scintillation light emitted from the absorbed x-rays.Because of the optical details involved with the light collection inthese detectors, the detector boxes need to have an aspect ratio inwhich the depth of the box cannot be much smaller than half the width ofthe box. Because the PMTs also typically protrude from the rear of theboxes, this means that the detectors must either be recessed in aspecial cabinet built into the side of the vehicle enclosure, whichrequires extensive modifications to be made, or the detectors need to beconcealed completely within the vehicle enclosure.

Both approaches require that any overlying material be sufficientlytransparent to the backscattered x-rays so that the x-rays can bedetected by the detector boxes. Because the energy of the backscatteredx-rays is substantially lower than the x-rays in the incident beam dueto the physics of the Compton Scatter process, this overlying materialmust have a relatively low atomic number, and is typically chosen to beplastic, thin aluminum (approximately 0.25-1.0 mm thick) or some kind ofcomposite carbon-based material. Since the material typically found inthe sides of vans and truck enclosures is steel, the steel must beremoved and replaced with more transparent material if the detectors areconcealed within the vehicle enclosure. This extensive modification iscostly, and also means that thermal insulation must be removed from theenclosure sides in the region where the detectors are positioned. Thematerial in the regions where the steel in the vehicle enclosure isremoved has been called the “scan panel.”

Around the year 2004, backscatter systems operating at a lower end-pointenergy of 225 keV were introduced. These systems had the detectorscontained completely inside the vehicle enclosure. Earlier systems hadcustom-made compartments mounted on the side of the vehicle chassis,with the backscatter detectors concealed inside. The material in frontof the detectors was thin aluminum. In more recent systems, steel isremoved from the side of the original vehicle enclosure and replacedwith a low atomic number composite “scan panel,” as describedpreviously. In both cases, the detectors are concealed within thevehicle enclosure due to their approximately 12″ depth.

As described previously, prior-art box-type backscatter detectors arebulky and cannot therefore be easily mounted to the exterior of avehicle for mobile imaging systems, particularly if the system needs tobe used covertly. They therefore need to be concealed within the vehicleenclosure or to be stored in an external recessed cabinet, as inprior-art systems. Both of these options require extensive modificationsto the vehicle enclosure as described above.

FIG. 11, in contrast to existing systems, illustrates a mobile x-rayscanning van 1102 with an external detector 1110 mounted on a side ofthe van. FIG. 11 illustrates how some embodiments disclosed herein arex-ray scanning systems that are mobile and employ detectors mountedexternally to a vehicle housing the scanner. A scanner such as thatillustrated in FIG. 1 or FIG. 3, for example, can be housed within thevan 1102.

The detector 1110 has a thin enough profile that it can be mounteddirectly onto the exterior of the vehicle enclosure, withoutmodification to the enclosure. The detector 1110 is “fixedly mounted,”meaning that it is configured to remain mounted to the exterior of thevan when the van 1102 is being driven. It need not be stowed duringtravel.

In some embodiments, the detector 1110 is less than 1 inch thick and canbe based on wavelength-shifting fibers (WSF). As known in the art, oneor more sheets of scintillating screen, such as GOS or BaFCI, can beinterspersed with one more more layers of WSF to extract thescintillation light. The fibers can be bundled, with one or more ends ofthe fiber bundle attached to a photodetector, such as a PMT or a solidstate device such as a silicon PMT. The detector 1110 is used as theprimary means of detecting backscattered x-rays from a target. Someembodiments can be designed to conform to the outer surface of thevehicle enclosure, allowing the system to remain essentially covert andnot affect the visual aesthetics of the vehicle.

In some embodiments, a low-profile WSF backscatter detector hasdual-energy capability. As known and practiced in the art, WSF detectorscan be designed to have two separate readout channels: one moresensitive to lower energy x-rays and the other more sensitive to higherenergy x-rays. Typically, the front channel is the low-energy channeland has scintillator optimized to absorb the lower energy range. Therear channel typically detects the higher energy x-rays that passthrough the low-energy channel, and is optimized for higher detectionefficiency at the higher x-ray energies. Quite often, an attenuatingfilter is placed between the two channels, such as a thin copper sheet,or similar. The ratio of the signals from the two channels can be usedto characterize the energy spectrum of the backscattered x-rays. Aspectrum with a relatively higher signal in the low energy channelcompared with the high energy channel can signify scatter originatingfrom low atomic number materials, such as organic objects. Conversely, aspectrum with a relatively lower signal in the low energy channelcompared to the high energy channel can signify scatter from materialswith higher atomic number, such as metallic objects.

To summarize, some advantages of the low-profile WSF detectors mounteddirectly onto the exterior enclosure of a mobile backscatter imagingplatform as described in this disclosure include the following:

-   1. No expensive modification of the vehicle enclosure and no scan    panel are required, substantially reducing overall system costs.-   2. X-rays entering the detector do not have to pass through the    enclosure material, which would otherwise attenuate the scattered    x-rays, increasing the signal-to-noise ratio of the resulting    backscatter images.-   3. Because the detectors do not have to be mounted inside the    enclosure behind a scan panel of low-attenuation material, the    detector area is not as constrained and the detectors can be    substantially larger, increasing the signal-to-noise ratio of the    backscatter images.-   4. Dual-energy capability can easily be added to WSF detectors,    which is not the case with the standard, existing, box-type    detectors. In addition, mounting the detectors outside the enclosure    increases the sensitivity of the low-energy channel due to reduced    attenuation of the low energy x-rays, enhancing the ability of the    detectors to perform physical material discrimination.

FIG. 12 is a flow diagram illustrating an embodiment method. At 1250, acollimated fan beam of incident x-ray radiation is produced. At 1252,rotation of a chopper wheel is effected, wherein the chopper wheel isconfigured to be irradiated by the collimated fan beam, in a rotationplane that is substantially non-perpendicular relative to a planecontaining the collimated fan beam of incident radiation.

It will be understood that rotation of the chopper wheel can also beeffected prior to producing the collimated fan beam of incidentradiation. Furthermore, in some embodiments, effecting rotation of thechopper wheel includes causing the rotation with an angle between therotation plane of the chopper wheel and the plane containing thecollimated fan beam of incident radiation of less than 30°. Theorientation of the fan beam and chopper wheel are further illustrated inFIGS. 3 and 4, for example. Furthermore, in some cases, the angle can beless than 15°. Effecting rotation of the chopper wheel can also includeusing a disk chopper wheel with a rim, a center, and one or more radialslits extending toward the rim of the disk and toward the center of thedisk, with the slits configured to pass x-ray radiation from thecollimated fan beam, as illustrated in FIGS. 3 and 4. Effecting rotationcan also include using the disk chopper wheel with one or more taperedslips having greater width toward the rim of the disk than toward thecenter of the disc, as illustrated in FIG. 9, for example. Effectingrotation can likewise include using the disk chopper wheel withchamfering on at least two edges, or on all edges, of the one or moreslits. Example chamfering is illustrated in FIG. 8.

Producing the collimated fan beam can include producing x-rays withend-point energies between about 50 keV and 500 keV. For example, x-rayswith endpoint energies between about 200 keV and 250 keV can be produced

In other embodiments, a procedure also includes detecting x-rayradiation backscattered by objects irradiated by the incident radiationhaving passed through the chopper wheel. For example, target objectsthat can be irradiated and the backscatter radiation detected caninclude the suitcase illustrated in FIG. 3 and the car illustrated inFIG. 1, for example. In some embodiments, detecting the backscatterx-ray radiation can include using one or more backscatter x-raydetectors mounted to an external surface of a vehicle, such as theexternal detector 110 in FIG. 1 or the detector 1110 in FIG. 11, forexample. The backscatter detectors can be fixedly mounted to theexterior surface of the vehicle, and the backscatter detector caninclude a WSF detector.

Embodiment systems with angled chopper disks as described in thisdisclosure can have the following advantages, for example, over existingsystems:

-   1. Greatly reduced weight, which is useful for high rotational speed    disks made of tungsten and for hand-held systems.-   2. Greatly reduced cost of the rotating disk, support bearings, and    driving motor.-   3. Faster spin-up times for the disk, useful for rapid deployment of    inspection vehicles.-   4. Lower gyroscopic effects of the spinning disk due to the greatly    reduced moment of inertia.

The teachings of any patents, published applications and referencescited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An x-ray scanning system comprising: an x-ray source configured to produce a collimated fan beam of incident x-ray radiation; and a chopper wheel, configured to be irradiated by the collimated fan beam, oriented in a wheel rotation plane relative to the collimated fan beam, an effective thickness of the chopper wheel being greater than an actual thickness of the chopper wheel as a function of the orientation of the chopper wheel relative to the collimated fan beam.
 2. The x-ray scanning system of claim 1, wherein the effective thickness is at least 25% greater than the actual thickness.
 3. The x-ray scanning system of claim 2, wherein the effective thickness is at least 50% greater than the actual thickness.
 4. The x-ray scanning system of claim 3, wherein the effective thickness is at least 100% greater than the actual thickness.
 5. The x-ray scanning system of claim 4, wherein the effective thickness is at least 400% greater than the actual thickness.
 6. The x-ray scanning system of claim 1, wherein an angle between the wheel rotation plane and a beam plane containing the collimated fan beam of incident radiation is less than 30°.
 7. The x-ray scanning system of claim 6, wherein the angle between the wheel rotation plane and the beam plane containing the collimated fan beam of incident radiation is less than 15°.
 8. The x-ray scanning system of claim 1, wherein the chopper wheel is a disk with a rim and a center, the disk including one or more radial slits extending toward the rim of the disk and toward the center of the disk, and the one or more slits being configured to pass x-ray radiation from the collimated fan beam.
 9. The x-ray scanning system of claim 8, wherein the one or more slits are tapered slits having greater width toward the rim of the disk than toward the center of the disk.
 10. The x-ray scanning system of claim 8, wherein the chopper wheel includes chamfering on at least two edges or on all edges of the one or more slits.
 11. The x-ray scanning system of claim 1, wherein the x-ray source is further configured to produce the collimated fan beam of incident x-ray radiation with end-point x-ray energies in a range between about 50 keV and 500 keV.
 12. The x-ray scanning system of claim 11, wherein the x-ray source is further configured to produce the collimated fan beam of incident x-ray radiation with end-point x-ray energies in a range between about 200 keV and 250 keV.
 13. The x-ray scanning system of claim 1, wherein the chopper wheel is configured to output a scanning pencil beam upon a rotation thereof.
 14. The x-ray scanning system of claim 1, the system further comprising one or more backscatter detectors configured to detect x-ray radiation backscattered by objects irradiated by the incident radiation having passed through the chopper wheel.
 15. A method of x-ray scanning, the method comprising: producing a collimated fan beam of incident x-ray radiation; irradiating a chopper wheel with the collimated fan beam; and effecting rotation of a chopper wheel in a wheel rotation plane that is oriented relative to the collimated fan beam such that an effective thickness of the chopper wheel is greater than an actual thickness of the chopper wheel.
 16. The method of claim 15, wherein the effecting rotation in the wheel rotation plane that is oriented relative to the collimated fan beam is such that the effective thickness is at least 25% greater than the actual thickness.
 17. The method of claim 16, wherein the effecting rotation in the wheel rotation plane that is oriented relative to the collimated fan beam is such that the effective thickness is at least 50% greater than the actual thickness.
 18. The method of claim 17, wherein the effecting rotation in the wheel rotation plane that is oriented relative to the collimated fan beam is such that the effective thickness is at least 100% greater than the actual thickness.
 19. The method of claim 18, wherein the effecting rotation in the wheel rotation plane that is oriented relative to the collimated fan beam is such that the effective thickness is at least 400% greater than the actual thickness.
 20. The method of claim 15, wherein effecting rotation of the chopper wheel includes causing the rotation with an angle between the wheel rotation plane of the chopper wheel and the beam plane containing the collimated fan beam of incident radiation being less than 30°. 